1. Field of the Invention
The present invention is directed to a local radio-frequency antenna for a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for, among other things, acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static, basic magnetic field that is generated by a basic field magnet system. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals, and picks up the generated magnetic resonance signals from which magnetic resonance images are produced.
The magnetic resonance apparatus has an imaging volume in which a region of the examination subject to be imaged is positioned for producing magnetic resonance images. Oriented at the transverse cross-section of maximum expanse of an average patient, many magnetic resonance apparatus have a spherical imaging volume with a diameter of approximately 40 cm. In typical applications, this imaging volume is rarely completely utilized at one time. The spatial expanse of the imaging volume and the degree of homogeneity of the basic magnetic field required therein determine the costs of the basic field magnet, and thus of the entire magnetic resonance apparatus to a considerable extent.
In magnetic resonance technology, a high homogeneity of the basic magnetic field is a decisive factor for the quality of the magnetic resonance images. Inhomogeneities of the basic magnetic field within the imaging volume cause geometrical distortions of the magnetic resonance image that are proportional to the inhomogeneities. Shim systems are utilized for improving the basic magnetic field homogeneity within the imaging volume. Passive shim systems and active shim systems are known.
In a passive shim system, a number of lamellae composed of a magnetic material, particularly a ferromagnetic iron alloy, are attached in a suitable arrangement in the examination space of the magnetic resonance apparatus. To that end, the basic magnetic field within the imaging volume is measured before the application of the lamellae. A computing program determines the suitable number and arrangement of the lamellae based on the measured values. U.S. Pat. No. 5,635,839 discloses a passive shim system for this purpose wherein shim boxes fittable with ferromagnetic lamellae are insertable into corresponding shim box receptacles of a gradient coil system of a magnetic resonance apparatus.
In active shim systems, shim coils that can be charged with direct currents are utilized for homogenizing the basic magnetic field. Among other things, an active shim system is employed for fine correction when extremely high homogeneity is required, for example in order to correct field distortions caused by the magnetic susceptibility of the examination subject at least partially disposed in the imaging volume.
It is also important for a high quality of magnetic resonance images that magnetic resonance signals be picked up be radio-frequency antennas of the radio-frequency system with a high signal-to-noise ratio. It is known to employ a radio-frequency antennas fashioned as a stationary antenna, referred to as a whole-body antenna, that can be utilized as a transmission antenna and as a reception antenna. Additionally, local radio-frequency antennas, referred to as local antennas, are utilized for improving the signal-to-noise ratio. These are antennas that are adapted to the size of a region to be imaged. For example, there are local antennas that are permanently installed in the support mechanism or that can be latched at fixed positions on the bearing mechanism, for example for examining a shoulder or a female breast. There are also freely movable local antennas, for example flexible antennas and extremity antennas. The local antenna can be fashioned as a pure reception antenna, with a whole-body antenna then being utilized as the transmission antenna.
Further, German OS 197 41 748 discloses a magnetic resonance apparatus wherein a shim coil is integrated in the housing of a local radio-frequency antenna.